Effects of nucleotides on expression of the folate sensitive fragile sites.

Further studies on the effect of nucleotides upon expression of the folate sensitive fragile sites have confirmed that high concentrations of thymidine (300 mg/l) but not BrdU (600 mg/l) induce these fragile sites. Induction by thymidine can be suppressed by addition of cytidine in a dose response manner. Studies on other nucleotides suggest that guanosine, but not adenosine or cytidine, can induce the fragile sites. The previously hypothesised model for the DNA at a fragile site as a naturally occurring polypurine/polypyrimidine sequence which becomes amplified is further developed.